The present invention relates generally to a multi layer electrolyte and a cell using the electrolyte, and more particularly to a multi layer electrolyte which is used in a secondary cell and which comprises a solid electrolyte and other electrolyte such as gel electrolyte and/or electrolytic solution layer laminated on the solid electrolyte.
Conventionally, materials which are soluble in electrolytic solution were not used as active materials for a positive electrode or a negative electrode of a cell. The main reasons why such materials cannot be used as active materials are considered as shown in the following three items.
1. Electronic conductivity is not retainable between active material dissolved into electrolytic solution and a collector. Therefore, reactivity of active material deteriorates.
2. Concentration of active material near the collector is decreased because the active material dissolves into electrolytic solution. Therefore, quantity of active material contributing oxidation and reduction reaction is decreased, and thereby capacity appearance rate, that is, ratio of actual capacity to theoretical capacity estimated from the quantity of active material is lowered.
3. Due to the dissolution of active material into electrolytic solution, electrode swells and thereby delamination occurs between a collector and the electrode.
In order to avoid dissolution of active materials into electrolytic solution, conventional methods generally adopt polymerizing of active materials, crosslinking of active materials, change of electrolytic solution such as using other solvent or solid electrolyte, and so on. Prior arts teaching such methods will be described below.
Japanese patent laid-open publication No. 7-134989 discloses a secondary cell using nonaqueous electrolyte system which uses carbon material as a negative electrode, and the surface of the carbon material is coated with solid polymer electrolyte film. In this publication, it is described that, by using carbon material whose surface is coated with solid polymer electrolyte film as a negative electrode, when, at charging such secondary cell, lithium ions which are solvated in electrolytic solution approach negative electrode, the electrolytic solution is cut off by the solid polymer electrolyte film coated on the surface of the negative electrode. Thus, only lithium ions pass through the solid polymer electrolyte film, and are intercalated between graphite layers of carbon material of the negative electrode.
Also, solid polymer electrolyte lithium secondary cell having solid polymer electrolyte layer between a positive electrode and a negative electrode is disclosed in Japanese patent laid-open publication No. 7-240233. In the solid polymer electrolyte lithium secondary cell disclosed in this publication, the negative electrode includes carbon material, solid electrolyte and electrolytic solution, and the electrolytic solution includes ethylene carbonate. In this publication, it is described that, by using a combination of the carbon material, the solid electrolyte and the electrolytic solution including ethylene carbonate in the negative electrode, this cell can have both useful discharge capacity and fine charge-discharge cycle characteristic.
Further, Japanese patent laid-open publication No. 7-320780 discloses a solid electrolyte secondary cell having a positive electrode, a negative electrode in which lithium is used as active material, and an electrolyte. The electrolyte is a solid polymer electrolyte comprising a composite of electrolyte salt and polymer, or is a gel polymer electrolyte which is made by impregnating a electrolytic solution comprising electrolyte salt and aprotic solvent into polymer. In this case, the polymer may be polyamide, polyimidazole, polyimide, polyoxazole, polytetrafluoroethylene, polymelamineformamide, polycarbonate, or polypropylene.
Further, Japanese patent laid-open publication No. 7-320781 discloses a solid electrolyte secondary cell having a positive electrode, a negative electrode in which lithium is used as active material, and an electrolyte. The electrolyte is a solid polymer electrolyte comprising a composite of electrolyte salt and polymer, or is a gel polymer electrolyte which is made by impregnating a electrolytic solution comprising electrolyte salt and aprotic solvent into polymer. In this case, the polymer may be a vinyl copolymer such as vinyl chloride-vinyl acetate copolymer, vinyl chloride-methyl acrylate copolymer, vinyl acetate-acrylonitrile copolymer, vinyl chloride-acrylonitrile copolymer, styrene-acrylonitrile copolymer, styrene-vinyl acetate copolymer, styrene-methyl methacrylate copolymer, and the like.
In the above-mentioned two Japanese patent laid-open publications Nos. 7-320780 and 7-320781, it is described, with respect to these solid electrolyte secondary cells, that since a solid polymer electrolyte or gel polymer electrolyte used in these secondary cells is hard to react with a negative electrode and since an internal resistance of the secondary cells is hard to increase even after charge-discharge cycles are repeated, it is possible to obtain superior charge-discharge cycle characteristics.
Japanese patent laid-open publication No. 9-330740 discloses a electrochemical cell comprising two kinds of different electrodes, wherein, after a solution which becomes a material of solid polymer electrolyte is impregnated into an electrode, the solution which becomes a material of solid polymer electrolyte is cured by heat treatment to solidify an electrolyte portion, and wherein the electrochemical cell is composed by combining electrodes via the solid polymer electrolyte therebetween. In this publication, it is described that, by such structure of electrochemical cell, it becomes possible to fabricate a lithium secondary cell having low internal resistance and high energy density.
However, techniques of polymerization or gelation of active materials described in the above-mentioned prior art publications have the following problems.
A first problem resides in that polymerization of active material monomer is not easily performed and it is difficult to synthesize large quantity of polymer.
A second problem resides in that polymerized active material often includes many and various impurities such as catalyst, initiator, oxidizing agent and by-product of reaction used or produced when active material monomer is polymerized. It is difficult to remove these impurities by purification. These impurities deactivate active materials that should contribute to oxidation-reduction reaction, and therefore the capacity appearance rate of cell is deteriorated.
A third problem resides in that it is difficult to mix, at molecule level, active material which is insolubilized due to polymerization with conduction assisting agent added when forming an electrode, and therefore the capacity appearance rate is deteriorated.
A fourth problem resides in that, by polymerizing active material monomer, oxidation-reduction potential shifts toward low potential side at the positive electrode and toward high potential side at the negative electrode and, therefore, there occurs a decrease in electromotive force.
A fifth problem resides in that, when crosslinking of active material is performed, reactivity of active material in oxidation-reduction reaction is deteriorated due to the change of molecular structure of active material, and thereby the capacity appearance rate is decreased.
Also, the techniques of changing electrolyte, that is, using other solvent, using solid electrolyte, and the like, described in the above-mentioned prior art publications have the following problems.
First, changing electrolyte causes deterioration of activity of active material, thereby the capacity appearance rate deteriorates.
Second, when solid electrolyte is used to avoid dissolution of active material into electrolytic solution, an equivalent series resistance (ESR) of a cell becomes high due to low ionic conductivity of the solid electrolyte. Therefore, capacity of a cell is decreased and it becomes impossible to quickly charge and discharge the cell.
Therefore, it is an object of the present invention to provide a multi layer electrolyte and a cell using the multi layer electrolyte which can overcome the above-mentioned disadvantages of the prior arts.
It is another object of the present invention to provide a cell which uses active material dissoluble in electrolytic solution as an electrode active material, and to provide electrolyte used for realizing such cell.
It is still another object of the present invention to provide a cell which can be quickly charged and discharged, and to provide electrolyte used for realizing such cell.
It is still another object of the present invention to provide a cell which has superior capacity appearance rate and superior cycle characteristics, and to provide electrolyte used for realizing such cell.
According to an aspect of the present invention, there is provided a multi layer electrolyte which comprises: a solid electrolyte layer; and at least one electrolyte layers selected from a gel electrolyte layer and an electrolytic solution layer and laminated on the solid electrolyte layer.
According to another aspect of the present invention, there is provided a cell including at least a positive electrode, a negative electrode and electrolyte. The electrolyte is a multi layer electrolyte which comprises: a solid electrolyte layer; and at least one electrolyte layers selected from a gel electrolyte layer and an electrolytic solution layer and laminated on the solid electrolyte layer.
According to still another aspect of the present invention, there is provided a cell including at least a positive electrode, a negative electrode and an electrolyte. The at least one of the positive electrode and the negative electrode includes active material which is dissoluble in electrolytic solution. The electrolyte is disposed between the positive electrode and the negative electrode and comprises a first electrolyte portion which includes solid electrolyte but which does not include gel electrolyte and electrolytic solution and a second electrolyte portion which includes gel electrolyte or electrolytic solution but which does not include solid electrolyte. The electrode or electrodes including active material which is dissoluble in electrolytic solution are adjacent the first electrolyte portion but are not adjacent the second electrolyte portion.
According to still another aspect of the present invention, there is provided a method for manufacturing a cell including at least a positive electrode, a negative electrode and multi layer electrolyte. The method comprises: preparing the positive electrode and the negative electrode, wherein at least one of the positive electrode and the negative electrode includes active material which is soluble in electrolytic solution electrolyte; forming a solid electrolyte layer on the at least one electrode including active material which is soluble in electrolytic solution; and opposing the positive electrode and the negative electrode via a separator including gel electrolyte or electrolytic solution so that the at least one of electrode including active material which is soluble in electrolytic solution opposes the separator including gel electrolyte or electrolytic solution via the solid electrolyte layer.
By using laminated or stacked electrolyte according to the present invention as electrolyte of a cell, it is possible to fabricate a cell which uses active material dissoluble in electrolytic solution. Such cell can be quickly charged and discharged. Also, such cell has high capacity and superior cycle characteristic, and can be quickly charged and discharged.